1. Field of the Invention
This invention relates generally to image sensors and the manufacture of image sensors having low dark current and more particularly to CMOS and CCD imaging sensors having low dark current characteristics.
2. Description of the Related Art
Modern digital cameras employ either CCD (charge coupled device) or CMOS (complementary metal oxide semiconductor) image capture sensors. CCD and CMOS technologies offer alternative methods for capturing images onto digital media.
The architecture of the CCD is largely devoted to light capture and processing is done mostly off-chip. By contrast, CMOS sensor architecture is more complex than CCD architecture. Within CMOS imaging sensors, each pixel cell typically includes a circuit that transforms photons from a photoactive-diode to a digital charge. With each pixel doing its own conversion, the chip can be built to require less off-chip circuitry for basic operation.
While CCD and CMOS architecture differs, both CCD (charge-coupled device) and CMOS (complimentary metal-oxide semiconductor) image sensors convert light into electrons using a plurality of photoactive-diodes, also known as photo-diodes, cells, or photo-sites.
The photo-diodes are generally arranged in a 2-D lattice. Each photo-diode in the lattice transforms light into an electron charge. Within the lattice, each photo-diode corresponds to at least one pixel in the captured image. Photo-diodes exhibit a photoelectric effect, characterized by the ability of certain materials to release an electron when impacted by protons, thereby creating a charge. The more photons impact a given photo-diode, the more charge builds up. Each diode is bordered by a nonconductive boundary, which forces the charge to build while the diode is exposed to light from a camera aperture. In essence, each of the photo-diodes acts as a bucket, tracking the number of incoming photons making contact with the photo-diode. The accumulated charge in each diode is measured and recorded as a corresponding brightness value.
FIG. 1 illustrates a cross-sectional view of a conventional CMOS image sensor 1. This CMOS image sensor 1 exhibits high dark currents and defects caused by ion-implantation. CMOS image sensor 1 includes anti-reflection layer 5, a hole accumulation diode 10. The hole accumulation diode 10 includes a sensing area 20. A The sensing area 20 has p-type implantation species, such as boron, along the interface between the sensing area 20 and anti-reflection layer 5. The hole accumulation diode 10 is highly doped with p-type impurities in-situ formed by an epitaxial growth process. Anti-reflection layer 5 serves to prevent incoming photons from reflecting off the surface of the photo-diode, and thereby failing to register a charge. The anti-reflection layer 5 may be comprised of silicon nitride (SiN).
FIG. 2 is a schematic diagram illustrating an energy band diagram of the stacked structure of conventional CMOS image sensor 1, described in FIG. 1. The horizontal axis correspond to increasing depth of the CMOS image sensor 1, beginning at anti-reflection layer 5, p-type implantation 15 area, and sensing area 20. The vertical axis represents the energy band. Dashed line 50 represents the interface between the undoped and doped portions of sensing area 20. Energy bands 55 represent the range of band gap 65, having a mid-gap 60. A hole accumulation layer is formed at the surface of the sensing area 20 due to p-type implant species in doped layer 15. As a result, the energy bands 55 bend upward as they approach the interface between the anti-reflection layer 5 and the sensing area 20, which is a key to a low dark current. On the other hand, defects are introduced at the surface of the sensing area by the implantation process (shown by X'es in FIG. 2). These defects are the origins of dark current.
To reduce the dark current in the photo-diode, it may be beneficial to reduce the number of electrons at interface 50, thereby reducing the number of electrons entering sensing area 20. The conventional CMOS image sensor 1 attempts to do this by introducing the doped layer 15, however, electrons can pass through doped layer 15.
The CCD and CMOS are manufactured via a wafer fabrication process by which different electrical components and structures are formed on the silicon wafers. Fabrication encompasses a plurality of stages, including deposition, photolithography, etching, ion implantation, and annealing. Conventional photo-diodes have p-type doping (usually Boron) and are grown upon the substrate material.
During the deposition stage uniform coatings of thin films are applied to the wafers. Materials such as silicon dioxide, silicon nitride and polycrystalline silicon can be deposited onto the wafers using a variety of techniques, such as evaporation, chemical vapor deposition and sputtering. In particular, photo-diodes can be generated by forming epitaxial silicon layers using a process known as chemical vapor deposition.
Photolithography and etching are the processes by which structures are created on the wafers. Photolithography commonly employs UV sensitive chemicals to form masks, which acts as stencils. Etching techniques are used to remove materials that decompose during the photolithography process.
The doping process introduces ions into the fabricated surfaces, thereby adding impurities and changing the electrical properties of the material into which the ions are implanted. During the doping process, the wafers are bombarded with ions which are thereby implanted into the silicon. The number of ions implanted via the bombardment process is controlled in order to produce surface layers with specific electrical properties.
Alternatively, an epitaxial layer can be doped during deposition by adding impurities to the source gas, such as arsine, phosphine or diborane. The concentration of impurity in the gas phase determines its concentration in the deposited film. As in CVD, impurities change the deposition rate.
In the annealing process, wafers are heated for a specific amount of time in a conditioned atmosphere (inert, oxidizing, reducing). This process serves to remove impurities (such as oxygen) from the surface layers and cause implanted ions to diffuse further into the silicon (called “autodoping”).
A common problem among imaging sensors is that even in the absence of light, some electrons will accumulate in the photo-diodes. This phenomenon is called “dark current.” Dark current within image sensors degrades the performance of the produced image. These dark currents are not generated by incoming photons, but are randomly generated by thermal excitation, current leaks within the imaging device, or from various other possible sources. When charges buildup in the photo-diode, the dark current is indistinguishable from charge resulting from the photoactive effect. This causes the dark current to effectively brighten areas of the captured image, and unevenly reduces contrast between dark and lighter areas of the image. Because dark current electrons are random with respect to each imaging device, their effects on each photo-diode is unpredictable, and thereby produces noise in the resulting image which is difficult to remove. Therefore, in order to provide clearer contrast and reliable color dark currents should be minimized.
The prior art attempts to address this problem by forming a hole accumulation diode using an ion-implantation. However, this method requires high temperature processes, and therefore narrows the options for manufacturing processes. In addition the implantation process itself causes defects in the sensing area of the imaging device.